1. Field of the Invention
This invention relates to a real image mode variable magnification finder used in a lens shutter camera, a still video camera, or the like.
2. Description of Related Art
In general, a finder optical system which is constructed independently of a photographing optical system as in a lens shutter camera is often used in a virtual image mode finder. The virtual image mode finder, however, has the problem that if it is designed so that a variable magnification ratio increases, the diameter of a front lens must be enlarged and the view of a visual field peculiar to the finder of this type becomes unclear.
A real image mode finder, by contrast, is such that the diameter of the front lens can be diminished because an entrance pupil can be located in the forward part of the finder. Furthermore, since an image formed by an objective system is observed through an eyepiece system, the defect of the virtual image mode finder is practically obviated, and thus a finder with a good view of a field frame can be obtained.
Today, many of lens shutter cameras having variable magnification functions employ such real image mode finders. However, if an attempt is made to attain a wide-angle finder with a variable magnification ratio in excess of 2, it will be imperative to use a three-lens unit zoom system. Such finders are available in various types. In particular, each of Japanese Patent Preliminary Publication Nos. Hei 5-34595, Hei 6-160709, and Hei 6-214159 discloses a finder in which an objective system is constructed with the three-lens unit zoom system composed of lens units of negative, positive, and positive powers.
The finder disclosed in Hei 5-34595 has a half angle of view of incidence of at least 30.degree. at the wide-angle position and a variable magnification ratio as large as about 2.5, which come under the classes of a wide angle of view and a high variable magnification ratio, respectively. The objective system of this finder particularly includes a second lens unit of a positive meniscus lens with a convex surface directed toward the object side and a third lens unit of a biconvex lens. With such a configuration, when the magnification of the finder is changed, ranging from the wide-angle position to the telephoto position, the amount of movement of the second lens unit increases in the vicinity of the wide-angle position. For this reason, a restriction on a driving mechanism for moving the lens is widened, and it becomes difficult that the magnification is changed in association with a photographic lens. Moreover, the finder has the drawback that its pupil diameter is as small as 4 mm, thus making it difficult to look through the finder.
The finder disclosed in Hei 6-160709 is designed to simplify a zoom mechanism in such a way that only the second lens unit of the objective system is moved. However, its variable magnification ratio is less than 2, and hence it cannot be positively said that this finder is satisfactory for practical use.
The finder set forth in Hei 6-214159 is such that its variable magnification ratio is more than 2.5, but the half angle of view of incidence is as small as nearly 25.degree. at the wide-angle position. Although this publication also discloses a finder with a half angle of view of incidence of more than 30.degree. at the wide-angle position, the variable magnification ratio of this finder is below 2, which is unsatisfactory for practical use.